Currently the `sync.Once` call is only used to init a Set in the add()
func. So when you add a value to a Set that is the result of one of the
Set operations (i.e. union, difference, intersect) the Set will be
reinitialised and the exiting values will be lost.
I don’t have a clue why this is showing up in my ACC tests just now, as
this code is in there for quite some time already. Somehow it seems to
have something to do with the refactoring of the helper/schema done
last week, as I cannot reproduce this with
47f02f80bc
Sander van Harmelen